Optical signals are used for a variety of purposes. For example, optical signals may be used in the field of communications, the signals being transmitted via optical fibers and various other optical equipment, such as switching equipment, multiplexers, amplifiers, and the like. Optical signals also are used in measurement systems, detector systems, and the like. There is a need to test the various optical fibers and other equipment. Often the testing involves the transmitting, reflecting or absorbing of light at one or more wavelengths and measuring light at one or more wavelengths.
Optical power meters (and many other light measuring instruments) generally do not discriminate wavelengths; rather they measure light intensity or optical power independently or substantially independently of wavelength. Therefore, it is necessary to associate a wavelength with the respective measurements. In prior light measuring instruments, which have used a scanning light source to provide incident illumination to a device under test (DUT), the source also has been used to provide to the measuring circuitry electrical signals that identify the wavelength of the light being supplied the DUT and, thus, allegedly then being measured. In some light measuring instruments the operator would manually provide an input to identify the wavelength of the light being supplied. A disadvantage to these approaches is the need for a direct relationship, indeed, a dependence, of the light source and the measuring circuitry so the electrical signals (from the light source) accurately identify wavelength and can be accurately construed (by the measuring circuitry) to identify wavelength. Although such direct relationship may not be difficult for light sources and measuring circuits manufactured by the same company, achieving such direct relationship is difficult and even may be impossible if the light source and measuring circuitry are manufactured by different companies. As improved and/or different light sources are and become available from a number of companies, such association becomes difficult, if not impossible, and may lead to errors in the output measurements of optical power/light intensity with respect to wavelength of the measured light. In an example using such a measuring instrument, the wavelength of incident light may be scanned over a range from 1520 nm (nanometers) to 1560 nm, over a period of about one second; and measurements may be made at a rate of from a few to 100,000 per second. Due to inaccuracy in coordination, such as are mentioned above, wavelength error on the order of hundreds picometers between the actual incident wavelength and the output data indicating optical power measurement made have been found to occur. There may be other larger or smaller inaccuracies in various measuring instruments. Thus, there is a need for better correlation or coordination of such intensity or optical power measurements with respect to the wavelength(s) of light at the point the respective measurements are taken.
Typically, to test optical components with narrow wavelength features, a tunable laser source is swept across a wavelength band, and the photonic performance of the component is monitored at a large number of wavelengths. In FIG. 1 an exemplary prior art optical test or measuring system 10 includes a tunable laser source (sometimes referred to as “TLS”) 11, a device under test (sometimes referred to as “DUT”) 12, an optical power meter (sometimes referred to as “OPM”) 13, and an output circuit and/or display 14, which may display the measurement data, use the measurement data, etc. Usually the optical power meter 13 itself and/or the photosensor thereof measures light power or intensity without regard to wavelength, and, therefore, coordination or synchronization of the tunable laser source 11 with the OPM 13 is necessary so the light power or intensity measurement data is coordinated with the wavelength of light provided the DUT 12 and being measured. More specifically, to create an accurate model of the performance of the DUT across the wavelength band over which the DUT is being tested, it is necessary to know the wavelength of light being measured for each measurement. In the past several techniques have been used to provide such coordination. In one conventional approach the TLS 11 is stepped to produce respective wavelengths at specific time periods, and the OPM 13 is coordinated with those time periods so the measurement at a given time period represents the measurement with respect to a corresponding output wavelength by the TLS. However, such systems are relatively slow and to an extent inaccurate because there is a need to provide a wait state or time period during each step while the TLS provides a particular wavelength output and the OPM makes the necessary measurement and because measurements are not made at wavelengths between respective steps. Accordingly, there is a need to improve the speed and accuracy or completeness of such optical measurements.
Another conventional approach to coordinating the measurements made by the OPM 13 with the wavelength of light from the TLS 11 has to been to provide tight synchronization between the TLS wavelength sweep with the measurement so the timing of the measurement can be used to determine the wavelength being presented at each measurement. Still another approach has been to provide direct so-called real-time communication between the TLS 11 and the OPM 13, with the TLS informing the OPM of the wavelength at each measurement time. However, both these approaches are somewhat inaccurate due to latency, that is, the delay in time required to communicate wavelength information from the TLS to the OPM and/or for the OPM to respond to the information indicative of the TLS wavelength. Another inaccuracy encountered in these and other systems may occur due to the non-linearity of the TLS wavelength sweep, that is, the change in wavelength of the TLS output over a period of time may be non-linear. Also, such non-linearity may be different for different laser sources (or other light sources) thus requiring complex and time consuming calibrations and also may vary with certain relatively uncontrolled conditions such as, for example, aging, various ambient conditions, such as temperature, pressure, humidity, and so forth. Although these approaches to coordination may be faster than the step coordination mentioned above, nevertheless, there is a need for improving the accuracy and reliability of the coordinating of the measurements made by the optical measurement systems with wavelength of the illuminating source.
The terms electromagnetic energy, light, optical, laser, laser output, and the like, are terms used herein. Such terms identify, broadly, electromagnetic energy that may be in part or all of the visible spectrum, the infrared, near-infrared and far-infrared spectra, and ultraviolet spectrum, and/or the like. Thus, reference to such terms indicates those wavelengths of electromagnetic energy operable in the context of the invention.
Accordingly, there is a need to obtain meaningful measurements of electromagnetic energy, e.g., with respect to wavelength, using a light source and measuring circuitry that are independent of each other.
There also is a need to improve the accuracy of optical power measurements with respect to wavelength of the incident electromagnetic energy.